Grain oriented electrical steel sheets are widely used mainly as iron core materials for transformers and electrical instruments. They are required to exhibit excellent magnetic properties, for example to be low in terms of iron loss value and high in magnetic flux density. In general, grain oriented electrical steel sheets are manufactured through the following steps. A slab with a thickness of 100 to 300 mm that has been controlled so as to have a predetermined chemical composition is heated to a temperature of 1250° C. or above and subjected to hot rolling, and the resultant hot-rolled sheet is annealed as required. Thereafter, the hot-rolled sheet or the hot-rolled and annealed sheet is cold rolled one time or is cold rolled two or more times with intermediate annealing performed in between, thereby forming a cold-rolled sheet with a final sheet thickness. Thereafter, the cold-rolled sheet is subjected to decarburization annealing. An annealing separator is then applied to the surface of the steel sheet, and the steel sheet is subjected to finish annealing for secondary recrystallization and purification.
That is, a general method for the manufacturing of grain oriented electrical steel sheets attains desired magnetic properties by the following treatments. First, a slab whose properties such as chemical composition associated with the formation of inhibitors have been appropriately controlled is heated to a high temperature in order to completely dissolve inhibitor-forming elements. Thereafter, the slab is hot rolled, subsequently cold rolled one time or two or more times, and further annealed one time or two or more times, thereby appropriately controlling the obtainable primary recrystallized microstructure. The steel sheet is then subjected to finish annealing where the primary recrystallized grains are secondarily recrystallized into {110}<001> oriented (Goss oriented) crystal grains.
In order to effectively promote the secondary recrystallization, firstly, it is important to control the precipitation state of a dispersed phase called an inhibitor such that the inhibitor will be dispersed uniformly with an appropriate size throughout the steel in order to suppress the growth (the normal grain growth) of the primary recrystallized grains during finish annealing. Then, of importance is that the primary recrystallized microstructure is formed of appropriately sized crystal grains with a uniform distribution across the sheet thickness. Typical inhibitors are substances exhibiting extremely low solubility in steel, with examples including sulfides, selenides and nitrides such as MnS, MnSe, AlN and VN. Grain boundary segregating elements such as Sb, Sn, As, Pb, Ce, Te, Bi, Cu and Mo are also used as inhibitors. In any event, controlling the behavior of inhibitors from the precipitation of inhibitors during hot rolling until the secondary recrystallization annealing is of importance in order to obtain a satisfactory secondary recrystallized microstructure. Such inhibitor control is becoming more important in order to ensure more excellent magnetic properties.
From the viewpoint of controlling inhibitor precipitation, a technique disclosed in Patent Literature 1 focuses on the influences of the temperature history from finish rolling to coiling in a hot rolling step on the magnetic properties of grain oriented electrical steel sheets. In a method according to this technique, a steel slab is hot rolled while controlling the finishing temperature (finishing delivery temperature) to be in the range of 900 to 1100° C., cooled under conditions such that the steel sheet temperature at a lapse of 2 to 6 seconds from the completion of the finish rolling satisfies Equation (1) below, and coiled at not more than 700° C.:T(t)<FDT−(FDT−700)×t/6  (1)
wherein T (t): steel sheet temperature (° C.), FDT: finishing temperature (° C.) and t: time (sec) after the completion of finish rolling in hot rolling.